What is the actual function of the shortwave IR in the wave machine process? Is it just another heat source?. i know it can apply a lot of heat in quick cycles and has ‘penetrating heat’ but is that it? seems a bit too simple.

A common bottomside heater configuration is IR-Convection-Convection. One of the big advantages of the IR zone is its ability to allow the flux carrier to spread and dry slowly before the board reaches the convections zones, which can blow wet flux carriers off specific areas of the assembly. Historically, the IR panels have been run rather hot, delivering lots of heat early in the preheat cycle. The subsequent convection modules could run a little cooler (although the temperature settings are not readily correlated) and soak the heat into the PWBs. In the case of long tunnel times, the assembler may reconsider the profile strategy, and turn the IR temperatures down, allowing the convection modules to provide the majority of the heat. Although two zones of preheat may appear to provide even less heating capability than a three-zone tin-lead process on what may already be a thermally challenging board, this is not necessarily be the case. [from the following http://www.practicalcomponents.com/whitepapers/Wave%20Soldering%20of%20High%20Complexity%20PWB%27s%20FINAL.pdf ]